The insulation between objects in inductively operating elements, for instance the insulation provided between two windings of a transformer, may degrade with time. One reason may be because this insulation, which in many high-voltage applications is made of paper or pressboard, is contaminated with a contaminant, for instance in the form of Copper Sulphide (Cu2S). In order to be able to provide suitable countermeasures, it is then of interest to determine the extent of degradation of the insulating ability. This may be done in order to know when the transformer is to be replaced and/or serviced. A better knowledge of the insulating ability can be of high economic importance. Through such knowledge it is furthermore easier to determine when a transformer is to be disconnected, which may also be of advantage from a safety point of view.
It is advantageous if this may be done without having to disassemble the transformer and directly inspect the insulating material. This is both cumbersome and time consuming. The transformer can not be used during such investigation. There is furthermore a risk that the transformer will be destroyed by the disassembly.
One type of such an inspection method that is frequently used nowadays is the dielectric frequency response method. With regard to this method some research has been carried out.
A lot of the literature in this area is directed towards a general description of the method, but without giving any real hints about how it should be applied practically.
Some examples are given below:
“Frequency response of oil impregnated pressboard and paper samples for estimating moisture in transformer insulation”, Chandima Ekanayake, Stanislaw M. Gubanski, Andrzej Graczkowski, Krzysztof Walczak, IEEE Transactions on Power Delivery, vol. 21, no. 3, July 2006, describes the frequency domain spectra of oil impregnated pressboard and paper samples, which can be used in modeling of results of diagnostic measurements in power transformers.
“Evaluation of Transformer Insulation by Frequency Domain Technique” P. K. Poovamma, A. Sudhindra, K. Mallikarjunappa, T. R. Afzal Ahamad, 2007 International Conference on Solid Dielectrics, Winchester, UK, Jul. 8-13, 2007, discusses measurements of dielectric response being used for assessing the paper-oil insulating system in transformers.
“Dielectric Response Analysis of Real Insulation Systems”, Uno Gafvert, 2004 International Conference on Solid Dielectrics, Toulouse, France, Jul. 5-9, 2004 discusses application of dielectric frequency response methods to a number of practically important real insulation system.
U.S. Pat. No. 7,292,048 describes a method and device for measuring a dielectric response of an electrical insulating system, where a first measurement result is determined by a frequency domain method and a second measurement result is determined by a time domain method. The first measurement result and the second measurement result are combined to form an overall measurement result as the dielectric response. The document is unfortunately silent regarding how the frequency domain method is actually used.
However, there exists one document, which does describe a practical solution to the problem of determining the degraded insulating ability of an insulation provided between two windings in a transformer. This document is U.S. Pat. No. 6,870,374.
U.S. Pat. No. 6,870,374 describes a method for identifying a type of abnormality in an insulation system of a power transformer, where dielectric losses in a section of the insulation system are measured, theoretical dielectric losses for the section are calculated based on the material properties, geometry, and temperature of the section, and a graphical representation of a percent difference between the measured and calculated dielectric losses is generated. A dielectric response signature is prepared from the DFR test on a transformer. The signature and measured response is compared with a modeled response of a transformer with a “normal” insulating structure and a library of signatures of known defects. Based on the comparison power factor defects in the insulation structure of the transformer under test can then be diagnosed.
The above mentioned method is also described in “Dielectric Frequency Response Measurement as a Tool for Troubleshooting Insulation Power Factor Problems”, Mark Perkins, Asim Fazlagic, George Frimpong, Conference Record of the 2002 IEEE International Symposium on Electrical Insulation, Boston, Mass. USA.
In view of what has been described above there is therefore a need for providing a different way to determine the change in insulating ability, which considers also the cause of the changes.